During normal operation of a wind turbine, power is supplied to the rotor by the wind, which turns a generator, either via a gearbox in geared machines or directly in direct drive machines. Output from the generator is rectified to DC (direct current) by a generator-side converter and stored transiently in a DC bus in a capacitive electric field. The DC bus energy is supplied to a line-side converter, which inverts the DC energy to AC (alternating current) at electrical grid frequency. Herein “electrical grid” or “grid” means an electrical power distribution system connected to the output of the line-side converter. This includes, for example, a collection system in a wind turbine farm that collects power from multiple wind turbines, and may be considered a local grid. The line-side converter produces both active power measured in megawatts (MW) and reactive power measured in mega volt amps reactive (MVAR). Active power must be supplied from the generator, but reactive power may be produced by the line side converter without generator action. When the line side converter produces no active power, but provides reactive power to the grid, or absorbs reactive power from the grid, it is operating as a local voltage regulator. By providing reactive power to the grid, it boosts the local grid voltage, and by absorbing reactive power from the grid it decreases the grid voltage. When a turbine is operating in this mode, it is described as acting in “synchronous condenser mode” or “STATCOM mode”. During this time, the generator and generator-side converter remain operative but are placed in a standby mode since they serve no function, and only the DC bus and the line-side converter are active.
During a low voltage condition on the grid, reactive current may be provided by the line-side inverter to support the grid voltage. Although purely reactive current does not transfer any net active or real power, it is not possible to provide reactive current without creating some active power losses, since all non-superconducting electrical components have series resistances. These deplete the voltage on the DC bus until a low DC bus voltage setpoint is met and the generator trips, which then may require several minutes to restart for diagnostics to be performed that no damage was done to the turbine associated with the trip. Real energy in the DC bus is described by E=½ C V2, where E is the energy, C is the DC bus capacitance, and V is the DC bus voltage. This energy is dissipated by losses in the system, so it must be replenished. When the wind turbine is producing power derived from the wind, this energy is obtained from wind power. However, during periods of low wind (below wind turbine cut-in speed) or high wind (above cut-out speed) or when needed by the system operator, it is sometimes desirable for wind turbines to operate as system voltage regulators without producing active power. When this occurs, DC bus energy must be supplied by the power system. If there is a 3-phase fault in the local power system, the system voltage drops to zero, so no power can be transferred. The DC bus energy is consumed in the process and the DC bus voltage drops. In this situation, the generator-side converter cannot replenish the DC bus to support real losses and there is a risk that the DC bus voltage will drop to unacceptably low levels or to zero, resulting in a turbine trip.